I know it's bad practise to reply to your own stuff but I've managed to fix my issue!
I created a shell script to "yum install" all the libraries in the pre-reqs list but duplicated it and shoved *.i686 on the end of the duplicates. I discovered I had a few 32 bit libraries still missing, and I also needed the -devel. versions of some.
There's also the following in my install notes:
yum install libXm.so.4
yum install libstdc++.so.6
yum install libgtk-x11-2.0.so.0 (64 bit RHEL ONLY)
yum install xulrunner.i686 (64 bit RHEL ONLY)
yum install libXtst*.i686
yum install PackageKit-gtk*.i686
yum install libcanberra-gtk2*.i686
yum install gtk2-engines.i686
yum install libaio*.i686
I may have duplicated a few from the documented pre-reqs but the ones above are also needed for the Installer to run correctly and not report errors with fonts and to get round "install mode" errors. The documentation needs to be a bit more specific as not all the libraries listed as pre-requisites are required in both x64 and x686 flavors - and as part of hardening for security certifications we have to remove as many unused libraries as possible.